A plasma welding process is classified in the category of non-consumable electrode welding processes, as well as a TIG welding process. The plasma welding process offers a greater heat concentration as compared to the TIG welding process, and thus is capable of a high speed welding with a narrow bead width and less distortion.
Moreover, the plasma welding process is capable of keyhole welding which is a one-side penetration welding process by using a plasma arc of a high energy density.
Examples of the TIG welding process are described in Patent Documents 1 to 3.
In the keyhole welding, a plasma arc pushes away the molten metal and penetrates the base material to thereby create a keyhole. This keyhole becomes a weld bead after the molten metal moves backward to create a molten pool along the wall as the weld travels further.
For this reason, the thickness possible with one-side one-pass welding of an I-type groove (square groove) butt is about 0.6 to 6 mm when using a carbon steel plate, and about 0.1 to 8 mm when using a stainless steel plate.
FIG. 1 schematically shows an example of a welding torch for use in such a plasma welding process.
Reference symbol 1 in FIG. 1 denotes a tungsten electrode. This tungsten electrode 1 is formed of rod-shaped tungsten with or without a small amount of an oxide of a rare earth element such as lanthanum oxide.
This tungsten electrode 1 is surrounded by an insert tip 2. This insert tip 2 forms a pipe-like shape and is provided coaxially to the tungsten electrode 1 with a gap therebetween. In addition, the insert tip 2 can be cooled by cooling water (not shown) circulating therein.
The insert tip 2 is further surrounded by a shield cap 3. This shield cap 3 forms a pipe-like shape and is provided coaxially to the insert tip 2 with a gap therebetween.
The structure is such that a center gas comprising an inert gas such as argon or helium is allowed to flow into the gap between the tungsten electrode 1 and the insert tip 2, and an outer gas comprising a mixed gas in which 3 to 7% by volume of hydrogen is added to an inert gas such as argon or helium, is allowed to flow into the gap between the insert tip 2 and the shield cap 3.
The center gas functions as a plasma gas and the outer gas functions as a shielding gas.
In addition, the structure is such that an electrical current from a pilot are power source 4 is applied to the tungsten electrode 1 and the insert tip 2 to ignite the preliminary plasma, and subsequently an electrical current from a main arc power source 5 is applied to the tungsten electrode 1 and a material 6 to be welded so that the plasma arc can flow from the tungsten electrode 1 to the material 6 to be welded.
Furthermore, the front end part of the tungsten electrode 1 is located inside from the front end part of the insert tip 2, so that it does not protrude to the outside of the front end part of the insert tip 2.
By so doing, the tungsten electrode 1 can be enclosed by the center gas comprising an inert gas, in a state of not being exposed to any oxidizing gas. Thus, the tungsten electrode 1 will not be oxidized nor exhausted upon welding. In addition, no sputter is generated, and long-term high quality welds can be produced. Furthermore, the running cost can be reduced.
For this reason, the plasma welding process has been used in a wide range of welding procedures mostly for producing pressure vessels, pipes, and joints.
However, in a conventional plasma welding process, it has been difficult to stably create a penetration bead upon welding of a stainless steel having a thickness of 8 mm or thicker, or a carbon steel having a thickness of 6 mm or thicker, which leads to a problem in that, due to the influence of gravity, the molten metal can not bear its own weight and the shape of the penetration bead can not remain stable. For this reason, a backing metal is applied to the backside of the part to be welded upon welding.
Moreover, if the penetration bead can not remain stable, the finish of the surface bead is affected, which leads to an inconvenient situation in that readjustment has to be done.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2003-311414    [Patent Document 2]: Japanese Unexamined Patent Application, First Publication No. 2006-26644    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-298963